The free-radical addition polymerization of vinyl monomers is known and has been extensively described (Ullmann's Encyclopedia of Industrial Chemistry, 2nd edn. vol. A21, 1992, 305 ff, VCH Weinheim). Polymerizations are associated with a considerable evolution of heat and an increase in the viscosity. The high viscosities can lead to problems of mixing and hence also of heat removal and reaction control.
Polymerizations are carried out in a stirred tank or in other reactors which exhibit mixing internals for commixing and for heat removal (Ullmann's Encyclopedia of Industrial Chemistry, 2nd edn. vol. B4, 1992, 87 ff, VCH Weinheim). However, in the case of reactions where a large quantity of reaction heat has to be supplied or removed, a process of this kind may prove difficult to carry out. This is so in particular if the fraction of solvents is low or if, indeed, a polymerization is carried out entirely in the absence of solvents.
Furthermore, reactors are known that are equipped with static mixing and cooling devices in such a way as to fill out their cross section. These static reactors, as they are known, are also operated as loop reactors in order to reduce the viscosity change within the reactor; in other words, a portion of the polymer solution is mixed with the feed, while another portion is taken from the reactor and polymerized further to completion in a downstream tube reactor as a plug flow. Reactors of this kind are described for example in EP 0 755 945 A1.
A simplification of this process consists in polymerization in a spiral coil heat exchanger (DE 199 15 916 A1) in which the tube volume does not exceed, generally, 10% of the total volume of the reactor. Disadvantages are the need for high pressures, and mixing problems at high viscosity.
It is known, moreover, to carry out polymerizations in a single-screw or twin-screw extruder, leading to advantages at high viscosity, described for example in U.S. Pat. No. 4,619,979 A. This process, however, leads to polymers having a relatively high gel fraction, of in some cases 55%. A further disadvantage is the low specific cooling area in comparison to that possessed by, for example, tube bundle reactors (EP 0 752 268 A1), thereby limiting the space-time yield.
Planetary roller extruders have been known for some time and were first used in the processing of thermoplastics such as PVC, for example, where their primary use is to charge downstream units such as calenders or roll mills, for example. As a result of their advantage of great surface renewal for material exchange and heat exchange, with which the energy introduced by friction can be rapidly and effectively removed, and also of the low residence time and narrow residence time spectrum, their field of use has expanded in recent times to embrace, among others, mixing and compounding operations which require a particularly temperature-controlled regime.
Planetary roller extruders exist, depending on the manufacturer, in different designs and sizes. Depending on desired throughput, the diameters of the roller barrels are typically between 70 mm and 400 mm.
Planetary roller extruders in existing design generally have a feed section and a process section, in which melting, cooling, mixing or compounding takes place.
The feed section consists of a conveying screw, to which all of the solid components are metered continuously. The conveying screw then passes on the material to the process section. However, there are also embodiments without a screw section, in which the planetary roller extruder is vertical and the material is fed directly between central (or sun) and planetary spindles.
The process section consists of one or more roller barrels, in series, located within which there is a driven central spindle (or sun spindle) and the planetary spindles. Roller sections, central spindle, and planetary spindles have a helical gearing which ensures material transport within the process section of the planetary roller extruder. The planetary spindles are driven by intermeshing by the central spindle. The rotational speed of the central spindle and hence the peripheral speed of the planetary spindles can be varied and are therefore an important parameter for controlling the operations.
The materials are circulated between central spindle and planetary spindles and/or between planetary spindles and helical gearing of the roller section in this way such that there is intensive material mixing and, respectively, effective heat exchange between the surfaces of the spindles and the roller barrels.
The number of planetary spindles rotating in each roller barrel can be selected and hence adapted to the requirements of the operation. The number of spindles exerts an influence on the free volume within the planetary roller extruder, and on the residence time of the material in the operation, and, additionally, determines the areal size for heat exchange and material exchange. With a constant roller barrel diameter, a greater number of spindles generally allows a better mixing action or a greater product throughput to be obtained. In this regime, however, the average residence time of the material is lower, which must be taken into account when carrying out residence time-orienting operations. The objective, therefore, is to adapt the fitting-out of the process section to the requirements of the operation in respect of thermodynamic requirements, efficiency, and product quality.
The maximum number of planetary spindles which can be installed between central spindle and roller barrel is dependent on the diameter of the roller barrel and on the diameter of the planetary spindles used. When using relatively large roller diameters, such as are necessary for achieving throughput rates on the production scale, or when using smaller diameters for the planetary spindles, the roller barrels can be fitted out with a greater number of planetary spindles. Typically, for a roller diameter of D=70 mm, up to seven planetary spindles are used, while for a roller diameter of D=200 mm, for example, ten spindles, and for a roller diameter of D=400 mm, for example, 24 planetary spindles can be used.
There are different embodiments of planetary spindles, which are adapted to the particular progress of operation. For instance, it may be advantageous to use planetary spindles which have interruptions over their periphery and, consequently, interrupt the inherently strict conveying characteristics of the helical gearing. The result of using this kind of spindle in one or more roller barrels is an increased axial transverse mixing, which allows the breadth of the molecular weight distribution to be reduced and the polymer to be adapted to the requirements.
Attention is drawn in this context to the patent applications and utility model DE 196 31 182 A1, DE 94 21 955 U1, DE 195 34 813 A1, DE 195 18 255 A1, and DE 44 33 487 A1, which represent an overview of the state of the art in the field of planetary roller extruders.
Thus, furthermore, DE 39 08 415 A1 describes the processing of rubber mixtures or rubberlike mixtures by means of planetary roller extruders. For the purpose of further processing on downstream devices, pre-batches or ready-produced mixtures are masticated and plasticated on a planetary roller extruder. Likewise described is the production of ready-produced mixtures in the planetary roller extruder. In this case, vulcanizing systems and other components are metered in to the rubber premixes.
DE 297 10 235 U1 discloses an apparatus for plasticating polymeric material, said apparatus being composed of at least two planetary roller extruders in parallel arrangement. The planetary roller extruders feed a common discharge stage, which in its turn may be a single-screw extruder, an inter-screw extruder or a gear pump. Between the planetary roller extruders and the discharge stage there may also be a devolatilizing unit, preferably composed of a downshaft acted on by vacuum.
In U.S. Pat. No. 3,825,236 A as well the use of a planetary roller extruder is shown, this extruder being located within a single-screw extruder. DE 23 03 366 A1 produces an extrudable composition comprising thermoplastic or thermoset polymer in screw extrusion presses with planetary rollers, the polymer, in the form of pellets or powder, after stuffing, being masticated and plasticated in the region of the planetary rollers and also compressed to extrusion pressure. Claimed therein as being essential to the invention is that, up until solid bodies are formed, the stuffed polymer is precompressed, then comminuted in the intake region of the planetary rollers, with pressure reduction, and also masticated, plasticated, and compressed to extrusion pressure.
DE 198 06 609 A1 discloses a process for the continuous solvent-free and mastication-free production of self-adhesive compositions based on non-thermoplastic elastomers in a continuously operating planetary roller extruder having a feed section and a compounding section, the compounding section of the planetary roller extruder being formed by at least one roller barrel.
The process is composed of the following steps:                a) feeding the solid components of the self-adhesive composition, such as elastomers and resins, into the feed section of the planetary roller extruder, where appropriate, feeding fillers, dyes and/or crosslinkers,        b) transferring the solid components of the self-adhesive composition from the feed section to the compounding section,        c) adding the liquid components of the self-adhesive composition, such as plasticizers, crosslinkers and/or further tackifying resins, to the compounding section,        d) preparing a homogeneous self-adhesive composition in the compounding section, and        e) discharging the self-adhesive composition.        
It is an objective of the present invention to provide a method for the continuous polymerization of vinyl monomers to vinyl polymers that does not have the disadvantages of the prior art, or at least not to the same extent.